The study of how afferent synaptic connections and activity influence neuronal development is central to further understanding of both neuroembryology and of the manner in which early sensory experience influences brain and behavior development. The proposed experiments will examine with light and electron microscopic methods the morphological organization and development of two avian brain stem auditory nuclei: nucleus magnocellularis and nucleus laminaris. The normal ontogeny of these neurons will be contrasted with their development after embryonic deafferentation or early restriction of the acoustic environment. This work will bring together manipulations of morphological and experiential variables in order to examine a wide range of afferent influences in the early life of two functionally-related neuronal types with many advantages for such studies. A major objective is to provide neuropathologic information on the central neural affects accompanying early acoustic deprivation or receptor damage. Several of the proposed experiments will involve early manipulations of the middle and inner ears, with subsequent study of morphological development in second-order and third-order auditory neurons in the brain. This project will provide a detailed and comprehensive survey of the effects on auditory neuron development of early conductive and sensorineural hearing loss.